1. Technical Field
The present application relates to systems and methods for combined radiotherapy and magnetic resonance imaging, particularly systems and methods that involve shielding magnetic fields and radiofrequency radiation from the radiotherapy and magnetic resonance imaging systems.
2. Related Art
A linear particle accelerator (also called a linac) is a type of particle accelerator used to accelerate subatomic ions at great speeds. Linacs are described, for example, by C. J. KARZMARK ET AL., MEDICAL ELECTRON ACCELERATORS (McGraw-Hill, Inc., Health Professions Division 1993), which is hereby incorporated by reference. Medical grade or clinical linacs (a.k.a. clinacs) accelerate electrons using a tuned-cavity waveguide in which the Radio frequency (RF) power typically creates a standing or traveling wave for the generation of high energy electrons or Bremsstrahlung X-rays for medicinal purposes.
Magnetic Resonance Imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body. MRI is described, for example, by E. MARK HAACKE ET AL., MAGNETIC RESONANCE IMAGING: PHYSICAL PRINCIPLES AND SEQUENCE DESIGN (Wiley-Liss 1999), which is hereby incorporated herein by reference.
It is desirable to be able to image with an MRI unit while being able to simultaneously perform radiation therapy using a linac. However, there are two major conflicts between the linac and MRI unit that should be overcome for these technologies to work together in a clinically acceptable way. The first problem is that the MRI unit's magnetic field accelerates charged particles in the linac by the Lorentz force on charged particles in a magnetic field determined by the equation F=q (v×B), where F is the force on the charged particle, q is the charge of the particle, v is the velocity, and B is the magnetic field. In linear accelerators, the electrons “ions” are typically generated by heating a thermionic material (a material where the electrons become detached when heated), which is the cathode, and when a positive voltage is applied to an anode (which is typically a wire grid), the electrons move from the cathode towards the anode. The anode is pulsed at 100's of megahertz such that the grouping of electrons pass thru the grid and on to be further accelerated. The cathode, anode, and later accelerating components form what is called the electron gun, and this gun can be shut down by an external magnetic field such that it will not produce electrons for further acceleration. The MRI magnet is usually shielded to reduce the magnetic field surrounding the magnet. Usually this magnetic fringe field remains above the level of the earth's 1 gauss magnetic field for a few meters from the MRI isocenter. The optimal distance for locating a linac near the patient is with the source at approximately one meter from the radiotherapy isocenter. For a system where the MRI and radiotherapy isocenters are substantially coincident, this puts the linac in a fringe field that could easily be on the order of 0.1 tesla (T, 1T=10,000 gauss) or higher. The magnetic field B vector is significant and oriented axial to the MR system (Z). The velocity v vector approaches the speed of light and is nominally at right angles (Y) to the B vector. The force F on the very light electron will accelerate the electrons perpendicularly out of their desired trajectory.
The second problem is that the high-powered RF source of the linac causes interference with the radiofrequency transmitter and receiver for signal detection in the MRI unit. The RF frequency transmit and (especially) receive coils employed are extremely sensitive and usually limited by thermal noise in the patient and RF coil structure. Gradient magnetic fields are used to set a range of frequencies around this central frequency to provide position information as a function of frequency. The high-powered RF source in the linac typically generates megawatt to tens of megawatt bursts of RF radiation tuned to the resonating cavity of the accelerator at several hundred Hertz during operation. This high-powered RF radiation is typically not on resonance with the MRI frequencies of operation, but has side bands at the MRI frequencies and can induce eddy currents in the conducting components of the MRI causing signal corruption or even damaging the MRI electronics. MRI systems usually include an RF shielded room to limit interference from external RF sources. The sensitive MRI receive-RF coils also need to be protected from the RF transmit field used for excitation. Usually this isolation is done with PIN diodes and/or back-to-back diodes, switching in/out tuned/detuned circuit elements that attenuate the RF induced signal. Further, it is important that the sensitive MRI pre-amps do not go into saturation with RF energy from any source.
U.S. Pat. No. 6,198,957 to Green, titled “Radiotherapy Machine Including Magnetic Resonance Imaging System” (hereinafter “Green”), teaches the combination of a MRI system and a horizontal linac. Green teaches that DC coils should extend around the horizontal linac to shield the MRI from magnetic fields produced by the linac and that DC coils should be used around the MRI to shield the linac from the leakage magnetic field of the MRI. Also, Green teaches that, for a linac that uses an electron beam, the main magnets of the MRI must be pulsed off while the electron beam of the linac is pulsed on. In an analogous way, PCT International Publication WO2004/024235 to Lagendijk et al., titled “MRI in Guided Radiotherapy Apparatus with Beam Heterogeneity Compensators” (hereinafter “Lagendijk”), teaches integrating DC coils into the design of the main magnet of the MRI to create a toroidal low field region outside the MRI to shield the linac electron gun source from the MRI leakage magnetic field. Lagendijk also teaches the design of a main magnet that provides limited shielding on the electron gun of the linac and allows higher fields along the accelerating path toward the linac target, though this permits further degradation of the beam and that requires correction with additional filters. Again, in a similar way, PCT International Publication WO2007/045076 to Fallone et al., titled “Integrated External Beam Radiotherapy and MRI System” (hereinafter “Fallone”), teaches that a shielding interface between the MRI and linac can be used if the linac and MRI are fixed to each other to allow shimming, as was disclosed by Green. Also, Fallone teaches the use of steering coils associated with the linac for detecting and correcting for deviations of the linac electron beam due to the magnetic field of the MRI. Finally, U.S. Patent Application Publication 2008/0208036 to Amies et al., titled “Combined Radiation Therapy and Magnetic Resonance Unit” (hereinafter “Amies”), teaches that the linac can be placed completely inside the MRI main magnet bore with the path of the accelerated electrons aligned with the main magnetic field lines, however, this shortens the distance of the linac from isocenter. This also limits the beam path to be exactly along the central axis of the magnet. In a horizontal bore magnet, the magnetic field lines begin to diverge away from the central axis as you approach either end of the magnet, and in so doing turn in a radial direction. Thus, the beam must be exactly along the central axis or else it will be effected by the radial components of the field toward the ends. The MRI also uses “pulsed gradient fields” which can also have significant radial components off the central axis. Each of these references also teach the shielding of the linac from the MRI magnetic field where shielding material is interposed or interfacing between the beam source and the patient.
Prototypes of the devices taught by Lagendijk (and related its applications) and Fallone have demonstrated that the shielding leads to very large devices that cannot fit in the standard linac (or clinac) room and present many technical challenges where significant compromises must be made in the quality of the radiotherapy that can be delivered, either requiring the radiotherapy devices to treat from large distances or through a large amount of material that can scatter and attenuate the beam, compromising the quality of the radiotherapy. Additionally, these prototypes have employed RF shielding boxes that completely enclose the MRI from the linac and the treatment room, making patient access an issue.
As will be appreciated, there exists a need for an improved solution to the shielding of an MRI and linac from each other that, among other things, mitigates the disadvantages of having to pass the radiotherapy beams through a large amount of material or from long distances.